This invention generally relates to downhole explosive devices and, more particularly, it concerns a downhole pressure attenuation device for use with perforating guns.
Conventional downhole explosive devices, such as, wireline or tubing conveyed perforating guns are employed during the completion of oil and gas wells to perforate the well casings and adjacent formations and in so doing bring the wells into production. In particular, a perforating gun having a plurality of spaced charges is lowered into a well bore in which a casing or screen has already been installed. The perforating gun is run downhole until the gun is located adjacent to the desired formation. A packer is set above the perforating gun and, then, the gun is detonated to create holes in the casing and surrounding formations and, thereby, release formation fluids or gasses into the well bore.
Typically, perforating guns forming part of a tool string are lowered into the well or casing using either a wireline or a tubing string. An example of a wireline or cable conveyed perforating gun and method is described in U.S. Pat. No. 2,155,322 issued to I. J. McCullough on Apr. 18, 1939. Tubing conveyed perforating guns and methods are described, for example, in U.S. Pat. Nos. 4,491,185 issued to G. B. McClure on Jan. 1, 1985, 4,512,406 issued to R. R. Vann et al on Apr. 23, 1985, and 4,790,385 issued to G. B. McClure et al on Dec. 13, 1988.
Tubing conveyed perforating guns and methods enjoy many advantages over wireline conveyed perforating guns. One advantage of using a tubing string to convey or lower the perforating gun into the well bore is that immediately after the perforating gun is detonated the well can be brought into production using the same tubing string used to convey the perforating gun as the conduit for bringing formation fluids to the surface.
Another advantage of using a tubing conveyed perforating gun system is that prior to detonation of the perforating gun the well bore can be cleaned or flushed of fluids and debris and trapped bottom hole pressures below the packer can be vented using tubing string vents located between the packer and the perforating gun. As described in above-mentioned U.S. Pat. Nos. 4,491,185, 4,512,406, and 4,790,385, the well bore is vented by opening a plurality of fluid vents, for example, in a perforated nipple or sub before the detonation of the perforating gun or guns to allow the well bore fluids to enter the tubing annulus and be removed at the surface.
Although the above-mentioned fluid vents in conventional tubing conveyed perforating gun systems do facilitate the flushing and/or venting of at least some well bore fluids prior to gun detonation, there is also a need for a pressure attenuation apparatus which can absorb, control, and/or reduce the peak pressure wave or quasi-static pressure pulse produced by downhole perforating gun detonation. The pressure pulses produced by downhole explosive device detonation have been known to unseat downhole packer plugs, damage seals, damage both mechanical and electrical downhole instrumentation, and collapse tubing, casings, and other downhole assemblies. Conventional, substantially mechanical tool string shock absorbers such as described, for example, in U.S. Pat. No. 4,693,317 issued to A. G. Edwards et al on Sept. 15, 1987, provide only a modicum of tool string protection from the enormous pressures and/or shock waves produced by downhole, perforating gun detonation.
In light of the foregoing, there is a need for a downhole device which can effectively attenuate the extreme pressures produced by the detonation of one or more perforating guns.